The present invention relates to an improved process for the production of selectively hydrogenated polymers of conjugated dienes utilizing a titanium hydrogenation catalyst and more particularly to the use of a new hydrogenation catalyst promoter.
The hydrogenation or selective hydrogenation of conjugated diene polymers has been accomplished using any of the several hydrogenation processes known in the prior art. For example the hydrogenation has been accomplished using methods such as those taught, for example, in U.S. Pat. Nos. 3,494,942; 3,634,594; 3,670,054; 3,700,633 and Re. 27,145, the disclosure of which patents are incorporated herein by reference. These methods known in the prior art for hydrogenating polymers containing ethylenic unsaturation and for hydrogenating or selectively hydrogenating polymers containing aromatic and ethylenic unsaturation, involve the use of a suitable catalyst, particularly a catalyst or catalyst precursor comprising a Group VIII metal.
In the methods described in the foregoing patents, a catalyst is prepared by combining a Group VIII metal, particularly nickel or cobalt, compound with a suitable reducing agent such as an aluminum alkyl. Also, while aluminum alkyls are the preferred reducing agents, it is known in the prior art that alkyls and hydrides of metals of Groups I-A, II-A and III-B of the Periodic Table of the Elements are effective reducing agents, particularly lithium, magnesium and aluminum. In general, the Group VIII metal compound is combined with Group I-A, II-A or III-B metal alkyl or hydride at a concentration sufficient to provide Group I-A, II-A and/or III-B metal to Group VIII metal ratios within the range from about 0.1:1 to about 20:1, preferably from about 1:1 to about 10:1. As indicated in the foregoing patents, the hydrogenation catalyst is generally prepared by combining the Group VIII metal compound and the reducing agent in a suitable solvent or diluent at a temperature within the range from about 20.degree. C. to about 60.degree. C. before the catalyst is fed to the hydrogenation reactor.
In 1985, Kishimoto et al. disclosed (in U.S. Pat. No. 4,501,857) that selective hydrogenation of the unsaturated double bonds in conjugated diolefin polymers could be achieved by hydrogenating such polymers in the presence of at least one bis(cyclopentadienyl)titanium compound and at least one hydrocarbon lithium compound wherein the hydrocarbon lithium compound can be an added compound or a living polymer having a lithium atom in the polymer chain. U.S. Pat. No. 4,980,421 discloses that similar hydrogenation activity can be accomplished with the same titanium compounds in combination with an alkoxy lithium compound which can either be added directly or as a reaction mixture of an organo lithium compound with an alcoholic or phenolic compound. The use of these catalyst systems was said to be advantageous because the catalyst were said to be highly active so that they were effective even in such a small amount as not to affect adversely the stability of a hydrogenated polymer and require no deashing step. Further, the hydrogenation was said to be able to be carried out under mild conditions.
In U.S. Pat. No. 4,673,714, bis(cyclopentadienyl)titanium compounds were disclosed which preferentially hydrogenate the unsaturated double bonds of conjugated diolefins but do not require the use of an alkyl lithium compound. These titanium compounds were bis(cyclopentiadienyl)titanium diaryl compounds. The elimination of the need for the hydrocarbon lithium compound was said to be a significant advantage of the invention disclosed in the '714 patent.
Copending, commonly assigned U.S. patent application Ser. No. 529,807, filed May 29, 1990, describes a process for the hydrogenation of conjugated diolefin polymers which first involves the polymerization or copolymerization of such monomers with an organo alkali metal polymerization initiator in a suitable solvent thereby creating a living polymer. The living polymer is terminated by the addition of hydrogen. Finally, selective hydrogenation of the unsaturated double bonds in the conjugated diolefin units of the terminated polymer is carried out in the presence of at least one bis(cyclopentadienyl)titanium compound preferably of the formula: ##STR1## wherein R.sub.1 and R.sub.2 are the same or different and are selected from the group consisting of halogen groups, C.sub.1 -C.sub.8 alkyl and alkoxy groups, C.sub.6 -C.sub.8 aryloxy groups, aralkyl, cycloalkyl groups, silyl groups and carbonyl groups. The hydrogenation step is carried out in the absence of hydrocarbon lithium and alkoxy lithium compounds.